1. Field of the Invention
The present invention generally relates to light emitting diodes, and more particularly to a fabrication method for light emitting diodes having a reflective layer to avoid light absorption by the diode's substrate.
2. The Prior Arts
FIG. 1a is a schematic sectional view showing a typical structure of a conventional light emitting diode (LED). As illustrated, the LED 100 contains a semiconductor substrate 103 and a light generating structure 102 on top of the substrate 103, and two ohmic contact electrodes 109 and 101 formed on the other side of the substrate 103 and on top of the light generating structure 102 respectively.
The light generating structure 102 is often made of layers of aluminum-bearing III-V compound semiconductors, such as AlGaAs for infrared and red lights, AlGaInP for yellow-green, yellow, amber, and red lights. The substrate 103 is usually made of gallium arsenide (GaAs) which has a matching lattice constant to that of the light generating structure 102. Lights generated by the light generating structure 102 are emitted toward all directions (i.e., isotropic). However, as the GaAs substrate 103 has an energy gap smaller than that of the visible light, a significant portion of the lights emitted by the light generating structure 102 is absorbed by the GaAs substrate 103, which significantly affects the LED 100's external quantum efficiency and, thereby, the LED 100's brightness.
FIG. 1b is a schematic sectional view showing another typical structure of a conventional LED. As illustrated, the LED 100′ requires etching part of the light generating structure 102′ so as to have the electrode 109′ configured on the same side of the LED 100′ as the electrode 101′. In addition, for the LED 100 of FIG. 1a, the substrate 103 has to be electrically conductive for the conduction of injection current between the electrodes 101 and 109, while, for the LED 100′ of FIG. 1b, the substrate 103′ could be electrically conductive or non-electrically conductive. Similar to the LED 100, the LED 100′ still suffers the same substrate absorption problem. For ease of reference, the LED 100 of FIG. 1a is referred to as having a vertical electrode arrangement, while the LED 100′ of FIG. 1b is referred to as having a planar electrode arrangement hereinafter.
Various approaches have been proposed to counter the problem of light absorption by the substrate. U.S. Pat. Nos. 4,570,172 and 5,237,581 disclose a similar light emitting diode structure as depicted in FIG. 1 except that, on top of the substrate, the light generating structure is sandwiched between a lower and an upper Distributed Bragg Reflectors (DBRs). By the configuration of the DBRs, lights emitted from the light generating structure toward the substrate are reflected and their absorption by the substrate is thereby avoided. However, the DBRs provide high reflectivity only for normal incident lights and the reflectivity decreases as the lights' incident angle increases. The improvement to the LED's external quantum efficiency and brightness is therefore limited.
U.S. Pat. No. 5,376,580 discloses another two approaches using wafer bonding processes. In one of the approaches, an LED epitaxial structure is first grown on a GaAs substrate. The LED epitaxial structure is then wafer-bonded to a transparent substrate. In the other approach, similarly, an LED epitaxial structure is first grown on a GaAs substrate. The LED epitaxial structure is then wafer-bonded to a mirror. Both approaches improve the LED's external quantum efficiency by removing the light-absorbing GaAs substrate, and letting lights either penetrate through the transparent substrate in the first approach or reflected by the mirror in the second approach. However, the problem with the approach using transparent substrate is that its wafer-bonding process requires to be operated under a high annealing temperature over an extended period of time, which would cause redistribution of doping profile and degrade the LED's performance. The problem with the approach using mirror is that the mirror's reflective surface is directly involved in the bonding interface during the wafer-bonding process, which would lead to roughness of the reflective surface or reactions and contaminations to the mirror's reflective surface.
Horng et al. discloses yet another technique in “AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding”, Applied Physics Letters, Nov. 15, 1999, Volume 75, Issue 20, pp. 3054-3056. In this technique, a Si substrate with an Au/AuBe reflector is fused to an LED epitaxial structure before removing the GaAs absorbing substrate. In general, Au/AuBe is used in AlGaInP LEDs to form ohmic contacts with p-type material. Here the Au/AuBe was used as a bonding layer and metal mirror in the wafer-bonded LED epitaxial structure. However, alloy material AuBe possesses inferior reflectivity and thereby limits the brightness improvement of the LED. The alloy process, which usually requires a high annealing temperature, would also compromise the surface flatness of the reflective mirror and degrade its reflectivity.
U.S. Pat. No. 6,797,987 discloses a light emitting diode also using a reflective metal layer. In the disclosed structure, in order to prevent the reflective metal layer from reacting with the light generating structure during the wafer bonding process, a transparent electrically conductive oxide layer such as ITO is interposed therebetween. To improve the ohmic contact between the ITO layer and the light generating structure, the disclosed structure proposes forming ohmic contact grid pattern or channels in the ITO layer, or forming an alloy metal mesh between the ITO layer and the light generating structure. The disclosed structure has rather complicated fabrication process, and therefore a high production cost. The alloy metal mesh requires a high temperature alloy process, and etching the alloy metal to form mesh is also very difficult to control. In addition, the thickness of the alloy metal requires special attention. If the alloy metal is too thin, the ohmic contact between the alloy metal and the light generating structure is inferior; if the alloy metal is too thick, the wafer bonding process couldn't achieve a strong bonding.